This invention relates to a superconductor and more particularly to a joint between two superconducting wires in a stabilized superconductor.
A typical superconductor is formed by embedding a superconducting wire capable of establishing a superconducting state at a cryogenic temperature within a stabilizer material for thermally and electrically stabilizing an established superconducting state. The materials used for the superconducting wire include an alloy material such as NbTi and NbTiTa as well as a compound material such as Nb.sub.3 Sn and V.sub.3 Ga. The most typical conductor comprises a superconducting wire formed of a number of NbTi filaments having a diameter of less than 50 microns and a stabilizer formed of a copper matrix.
The method of manufacturing the above-described superconductor will now be explained taking a Cu/NbTi superconductor (copper cladded NbTi superconductor) as an example. First, a number of copper-cladded NbTi bars are inserted into a copper tube having a typical diameter of from 100 mm to 250 mm. This assembly is used as a composite billet to be extruded into a composite rod having a diameter of from 30 mm to 80 mm which is then subjected to swaging, drawing or rolling for reducing the cross-sectional area and, after twisting, finished into the desired predetermined dimensions. This process is applied not only to superconductors including Cu/NbTi superconducting wires but also to other superconductors. The length of the superconductor is limited due to the limited volume of the composite billet used to from the superconducting wires.
On the other hand, as a stabilizer used for the purpose of thermal and electrical stabilization, copper or aluminum is used in a composite state with the superconducting wire. Recently, as superconducting solenoid magnets are put into practical use, superconductors are required to carry higher-density current and to be more compact and reduced in weight. Superconducting magnets for use in elementary particle detectors are further required to have high permeability with respect to elementary particles. Aluminum, particularly high purity aluminum, has superior electrical and thermal conductivity at cryogenic temperatures and, moreover, has good permeability and small specific weight. Aluminum further exhibits saturation characteristics in magnetic reluctance, providing a number of advantages against copper as a stabilizer material.
However, it is very difficult to make a superconductor having a stabilizer of aluminum, because the mechanical properties of high-purity aluminum are very different when compared to the superconductor material. For this reason, it is very difficult to make a composite material with these materials, and the high-purity aluminum stabilizer is generally combined after the copper cladded NbTi superconducting wire has been manufactured.
One example of a cross section of a superconductor thus manufactured is illustrated in FIG. 1, in which a conventional superconductor comprises a Cu/NbTi superconducting wire 1 which is a copper-cladded NbTi wire, and a stabilizer 2 of high-purity aluminum surrounding the superconducting wire 1. The Cu/NbTi superconducting wire 1 is embedded within the aluminum stabilizer 2. and they are metallurgically joined so that good electrical and thermal conduction is established therebetween. When a large superconducting solenoid magnet is to be manufactured, the superconductor to be wound must be long, and while the high-purity aluminum stabilizer 2 can be made as long as desired since the high-purity aluminum stabilizer 2 can be connected by hot extrusion, the length of the Cu/NbTi superconducting wire 1 of Cu/NbTi is limited. However, it is impossible to connect the superconducting wires 1 without any harm to the current characteristics. Therefore, the superconducting wires 1 has to be connected with predetermined lengths of the superconductors overlapping each other and with the high purity aluminum stabilizers 2 welded to each other. This process is disclosed in an article entitled "Cooling and Excitation Tests of a Thin 1 m.times.1 m "Superconducting Solenoid Magnet" by H. Hirabayashi et al, Japanese Journal of Applied Physics, Vol. 21, No. 8, August, 1982, pp. 1149-1154. A cross section of the joint of a superconductor thus manufactured is as shown in FIG. 2, in which two sections of the high-purity aluminum stabilizer 2 are welded together by a weld 2a.
In the above superconductor, since the shape of the superconductor is different from other portions at the joint and has a thickness twice as thick as the other portion, gap regions or portions from which the superconductor is absent are formed between the turns of a superconducting magnet an indirect cooling structure. This causes problems in that the depleted region is mechanically unstable and destroys the uniformity of the magnetic field.